Build a working cell, obtain a meaningful voltage and investigate one change without introducing uncontrolled differences.
Use a high-resistance voltmeter
EMF is the potential difference when negligible current is drawn. A high-resistance voltmeter approximates this condition and reduces changes caused by the cell discharging during measurement. A low-resistance load measures terminal voltage under current, which can be smaller because of internal resistance and polarisation.
Prepare clean metal electrodes and the appropriate metal-ion solutions in separate labelled beakers. Remove oxide from suitable metal strips with abrasive paper, rinse and handle consistently. Join the half-cells with a fresh compatible salt bridge. Connect the meter, note which electrode is attached to the positive terminal and record the sign as well as the magnitude. Wait for a stable reading.
For standard-condition comparisons use the specified ion concentrations and temperature; a room-temperature classroom reading is not automatically E°. Actual ion activities and junction effects can still differ from the simple ideal model.
Plan a concentration investigation
Keep one half-cell unchanged and vary the concentration in the other using accurately prepared dilutions. Maintain temperature, electrode materials and surface preparation, immersion arrangement, bridge electrolyte and meter. Use fresh solutions and a fresh or thoroughly rinsed bridge to reduce cross-contamination.
For example, hold Cu²⁺ concentration fixed and vary Zn²⁺ concentration in a zinc–copper cell. Predict that increasing Zn²⁺ will lower the forward cell EMF, then compare with measurements. Repeat each condition and report variation. An observed trend is stronger evidence than a single reading.
Calculate each diluted concentration using c₁V₁ = c₂V₂. To prepare 50.0 cm³ of 0.200 mol dm⁻³ solution from 1.00 mol dm⁻³ stock, transfer 10.0 cm³ and dilute to a final volume of 50.0 cm³; do not add 50.0 cm³ of water.
Record the sign and the conditions
An original illustrative result is +1.07 V for copper connected to the positive lead and zinc to the negative lead. Against a reference value of +1.10 V, the absolute difference is 0.03 V and the relative difference is 0.03/1.10 × 100 = 2.7%. This calculation describes the discrepancy; it does not identify its cause.
Record solution concentrations, temperature, electrode pair, voltmeter resolution and repeats. If the display is negative, first check the leads and written cell orientation. Do not silently replace a negative reading by its magnitude or claim the reaction direction from the sign without specifying the connections.
| Observation | Possible cause | Useful check |
|---|---|---|
| Near-zero or unstable reading | Bridge not contacting both solutions, poor electrical contact or dirty electrodes. | Check the ionic and external circuits separately. |
| Expected magnitude but opposite sign | Leads or cell order reversed. | Record which half-cell is on the positive lead. |
| Drifting voltage | Concentrations changing, contamination or substantial current drawn. | Use a high-resistance meter and consistent fresh solutions. |
| Reproducible offset | Non-standard concentrations/temperature or junction effects. | Compare conditions with those of the reference data. |
Improvements must target the error
Cleaning an electrode addresses a coating that hinders charge transfer; it does not correct the concentration of its solution. Repeats reveal random variation but do not remove a systematic calibration error. A thermostatically controlled bath is useful when temperature differences are a concern.
Use eye protection and the school’s assessment for metal salts. Collect metal-containing waste as instructed. Never short-circuit a commercial battery as a way to test its EMF; this practical uses appropriate low-energy laboratory half-cells.
In June 2023 Paper 1 Q02, the salt bridge and concentration effects were assessed. The key distinction is between ionic conduction that completes the circuit and electron transfer at the electrodes.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why should the voltmeter have high resistance?Show answer
It draws negligible current, giving an approximation to open-circuit EMF and limiting discharge-induced concentration changes.
Q2. How much 0.800 mol dm⁻³ stock is needed for 100 cm³ of 0.120 mol dm⁻³ solution?Show answer
Vstock = 0.120 × 100/0.800 = 15.0 cm³. Dilute that aliquot to 100 cm³ total.
Q3. A meter reads −1.08 V for a cell expected to have about 1.10 V magnitude. What should you check first?Show answer
The lead connections and the written cell orientation. Reversed connections reverse the sign.
Q4. Does repeating a reading remove a calibration offset?Show answer
No. It helps assess random scatter. Compare with a calibrated reference or correct the instrument calibration to address a systematic offset.
Q5. What happens if the bridge electrolyte precipitates one of the cell ions?Show answer
It changes that ion’s available concentration and may obstruct contact, so the measured voltage no longer represents the intended half-cell conditions.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 physical chemistry specification — 3.1.11 coverage and required skills.
- Chemrevise: Electrode potentials and electrochemical cells — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 1 mark scheme — Q02 p13 and Q08 p26; report pp3, 5–6: bridges, reaction direction, fuel-cell supply and comparison of fuels.
- AQA June 2023 Paper 1 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
- AQA practical handbook — Required practical 8: half-cell construction and EMF measurement; handbook pp126–128.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.